1. Field of the Invention
This invention relates to a useful, competitive commercial design that must be used for solid oxide electrolyte fuel cells to be a cost effective power source. The present invention relates to the arrangement of fuel cell module components necessary to construct an array of solid oxide fuel cells for large stationary power applications. The invention includes the definition of the sub components necessary to implement the invention.
2. Description of the Prior Art
High temperature solid oxide electrolyte fuel cells (SOFC) have demonstrated the potential for high efficiency and low pollution in power generation. Successful operation of SOFCs for power generation has been limited in the past to temperatures of around 1000° C., due to insufficient electrical conduction of the electrolyte and high air electrode polarization loss at lower temperatures. U.S. Pat. Nos. 4,490,444 and 5,916,700 (Isenberg and Ruka et al. respectively) disclose one type of standard, solid oxide tubular elongated, hollow type fuel cells, which could operate at the above described relatively high temperatures. In addition to large-scale power generation, SOFCs which could operate at lower temperatures would be useful in additional applications such as auxiliary power units, residential power units and in powering light-duty vehicles.
At the outset, it should be noted that due to the interplay of components and multiple views in various figures, there will be some shifting between figures, to better understand the prior art and the invention. Solid oxide electrolyte fuel cell (SOFC) generators that are constructed in such a way as not to require an absolute seal between the oxidant and the fuel streams, and presently use closed ended fuel cells of circular cross section, are shown in FIG. 1 of the drawings, with the closed end at the base of the SOFC Generator as shown in FIG. 5 of the drawings. Air flows inside the tubes on the cathode and fuel flows outside on the anode. The reaction of the fuel at the anode involves relatively pure fuel, for example, a mixture of hydrogen and carbon monoxide. To provide such fuel, feed natural gas can be catalytically reformed using, for example, nickel at a variety of locations, such as outside the SOFC generator or even, in-situ on the anode, as taught by Somers et al. in U.S. Pat. No. 4,374,184. On the other hand, air passes through a ceramic feed tube, exits at the end of a ceramic cell and reverses flow to diffuse through the inner fuel cell ceramic air electrode. Generally, the air is preheated by an exterior preheater or by an interior recuperator near a combustion area, such as described by Makiel in U.S. Pat. No. 4,520,082 and Draper et al. in U.S. Pat. No. 7,320,836. In these cells, interconnection, electrolyte and fuel electrode layers are deposited on an extruded and sintered lanthanum manganite air electrode tube by plasma spray or other techniques. In one embodiment, a lanthanum chromite interconnection is in the form of a narrow strip that runs axially over the entire active length of the air electrode tube; a yttria stabilized zirconia solid electrolyte is deposited in such a way as to almost entirely cover the air electrode tube, where this yttria stabilized zirconia does not become an active electrolyte until a temperature over about 700° C. is achieved in the fuel cell; and the electrolyte layer contacts or overlaps the edges of the interconnection strip leaving most of the interconnection exposed. Because the interconnection and electrolyte layers are dense, an overlap feature can provide a seal that prevents high temperature leakage of fuel.
In this embodiment, a nickel/yttria stabilized zirconia cermet, fuel electrode anode layer is deposited in such a way as to almost entirely cover the electrolyte, but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode, where this margin prevents shorting of the cell; and series electrical connection between cells is accomplished by means of a structure made from nickel mesh, or, more recently, nickel foam and nickel screen, as shown in U.S. Pat. No. 7,157,172 B2 A1 (Draper et al.). The foam part of the connection becomes sintered to the interconnection while the screen part becomes sintered to the fuel electrode of the adjacent cell. Problems associated with the tubular cell, are limited power density, long current path, and potential bowing along its length during and after sintering.
Another cell geometry has been disclosed or patented in which the lanthanum manganite air electrode has the geometric form of a number of integrally connected elements of triangular or “delta” like cross section, see FIG. 2 of the drawings. These triangular, elongated, hollow cells have been referred to in the prior art in some instances as Delta X cells where Delta is derived from the generally triangular shape of the elements and X is the number of elements. These type cells are described for example in basic, Argonne Labs U.S. Pat. No. 4,476,198; and also in U.S. Pat. No. 4,874,678; U.S. Patent Application Publication U.S. 2008/0003478 A1, and International Publication No. WO 02/37589 A2 (Ackerman et al., Reichner; Greiner et al., and Thomas et al. respectively). An encyclopedic publication by N. Q. Minh, in “Ceramic Fuel Cells”, J. Am. Ceramic Soc., 76 [3] 563-588, 1993 describes in detail a variety of fuel cell designs, including the tubular and triangular and other types, as well as materials used and accompanying electrochemical reactions.
Generally, in newer triangular, tubular, elongated, hollow cross-section, so called delta or Delta X cells, the resulting overall cross section has a flat face on the interconnection side and a multi-faceted triangular face on the anode side. Air-flows within the internal discrete passages of triangular shapes where, at the end of the cell, the air can reverse flow to diffuse through the porous air electrode if air feed tubes are used. In the Greiner et al. publication, providing cell end closure, above, a transverse channel is used to cause reverse flow so air passes down one channel and up an adjacent one so air feed tubes can be eliminated. The fuel channels are built into multiple adjacent units of the triangular tubular type cells, and provide better fuel distribution and equal cross-section of air and fuel channels.
In the triangular tubular, elongated, hollow, so called delta or Delta X cells, a dense lanthanum chromite interconnection covers the flat face. A yttria-stabilized zirconia electrolyte usually covers the multifaceted triangular face and overlaps the edges of the interconnection but leaves most of the interconnection exposed. A standard nickel/yttria stabilized zirconia fuel electrode usually covers most of the electrolyte but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. Series electrical connection between cells can be accomplished by means of a flat nickel felt or nickel foam pads, one face of which is sintered to the interconnection while the other face is sintered to apexes of the triangular multifaceted fuel electrode face of the adjacent cell. This felt or foam also aids in combination cell-to-cell connector thermal expansion control properties.
Most of these designs utilize ceramic air feed tubes, which present their own set of issues, since it is difficult to manufacture long, completely round and straight ones. This in turn can create problems of binding when insertion into the air feed channel of the cells is attempted.
Because of their large active area, triangular, elongated, hollow, seal-less cells, shown in FIG. 2, operate at higher current than cylindrical cells and stack packing density is improved. Relative to cylindrical cells, triangular tubular cells achieve less ohmic resistance, therefore cell voltage can be closer to theoretical Nerst potential; however, tubular fuel cells are easier to manufacture, are more robust and both are useful in the current inventive design. The triangular, elongated, hollow cell, in particular, because of its thin triangular cross-sectional configuration poses particular difficulties in sealing, at open ends, and in providing transverse recirculation gas extraction.
Other tubular, elongated, hollow fuel cell structures are described by Isenberg in U.S. Pat. No. 4,728,584—corrugated design, and in U.S. Patent Application Publication U.S. 2008/0003478A1 by Greiner et al.—triangular, quadrilateral, oval, stepped triangle and a meander; all herein defined as “hollow elongated tubes”. FIG. 3 describes a hybrid transitioned fuel cell with a flattened open non-active cross section and a triangular (Δ cross-section) merged and, morphed onto each, another SOFC design possibility, as described in U.S. application Ser. No. 12/169,261, filed on Jul. 8, 2008.
U.S. Pat. No. 6,656,623 (Holmes et al.) illustrates, in FIG. 5 of this application, a standard SOFC configuration with closed end down fuel cells and oxidant entry through the top of the generator, as has been the case since the basic Isenberg concept of U.S. Pat. No. 4,490,444, and is still the state of the art. Also, more clearly shown is a cut-away section of one fuel cell showing the end of the cell and an air feed tube. At the top of the fragile ceramic fuel cells, heavy exhaust ducts and air inlet plenums feeding to oxidant feed tubes are shown, all requiring very substantial support to not harm the closed end down SOFC tubes. The same is particularly true with U.S. Pat. No. 4,664,986 (Draper et al.), where massive metal overhead conducts are taught. U.S. Pat. No. 5,741,605 (Gillett et al.) illustrates a massive top weight oxidant distribution assembly on top of the bundles, and a massive oxidant inlet channel at the top of the SOFC stack.
Also, U.S. Pat. No. 4,801,369 (Draper et al.) teaches an apparatus having SOFC operating in a mode where the fuel cells are taught closed end up, at the top of the generator, and open end down, but the cells are operated by applying electricity to electrically dissociate water into H2 and O2. Here, electrons are not generated as in the SOFC mode, but are fed to generate O2 in an electrolyzer mode—which is the exact opposite of a fuel cell mode where electrons are generated. In Draper et al., O2 is discharged as the main product via a duct at the bottom of the generator, rather than providing air or O2 in to be reacted within the fuel cell to exit as depleted oxidant, steam is added at the bottom of the generator as well as through a line (not shown) at the bottom of the generator. This pure steam flow is added to provide a buffer zone or seal between oxygen and hydrogen regions. Also, in addition to gas seals, Draper et al. did not address thermal expansion of large units. Electricity input between electrodes causes the steam to dissociate into H2 and O2. This is non-equivalent in any sense to providing H2 and O2 to provide electricity.
U.S. Pat. No. 7,364,812 (Taylor and Zymboly) also utilize inverted fuel cells, but in a SOFC mode, shown in FIG. 8. There fuel is reformed in special fuel feed tube assemblies involving feed fuel reverse flow to contact interior nickel catalyst, rather than free flow from entry plenum to fuel cell outside surface. Three horizontal seal locator/plenum separation strips are utilized to connect to adjacent bundles.
Reiterating, solid oxide electrolyte fuel cell (SOFC) generators usually include a gas-tight, thermally insulated external container which houses individual chambers including a fuel cell chamber and a combustion chamber. The fuel cell chamber, in which power generation occurs, contains a solid oxide fuel cell stack which is made up of an array of series-connected solid oxide fuel cells, with associated fuel and air distribution means. The solid oxide fuel cells contained in the generator chamber can take on a variety of well known configurations, including tubular, flat plate, and corrugated or delta, etc. designs.
More specifically, FIG. 1 shows a prior art hollow elongated tubular solid oxide fuel cell 10, which operates primarily the same as the other designs that are discussed later but will be described here in some detail, because of its simplicity, and because its operating characteristics are universal and similar to both flattened and tubular, elongated hollow structured fuel cells such as triangular and delta SOFC's. Most components and materials described for this SOFC will be the same for the other type fuel cells shown in the subsequent figures. A preferred SOFC configuration has been based upon a fuel cell system in which a gaseous fuel F, such as hydrogen and carbon monoxide derived from reformed pipeline natural gas, is directed axially over the outside of the fuel cell, as indicated by the arrow F. A gaseous oxidant, such as air or oxygen O, is fed preferably through a hollow air/oxidant feed tube, here called air feed tube 12, concentrically positioned within the annulus 13 of the fuel cell, and extending near the closed end of the fuel cell (not shown, but closed end seen in FIG. 5 at 92), and then out of the air feed tube back down the fuel cell axially over the inside wall of the fuel cell, while reacting to form depleted gaseous air or oxygen, as indicated by the arrow O′ as is well known in the art.
Returning to FIG. 1, the prior art solid oxide fuel cell shown comprises a hollow elongated tubular air electrode 14 (or cathode). The air electrode 14 may have a typical thickness of about 1 mm to 3 mm. The air electrode 14 can comprise doped lanthanum manganite having an ABO3 perovskite-like crystal structure, which is extruded or isostatically pressed into tubular shape or deposited on a support structure metal or ceramic and then sintered.
Surrounding most of the outer periphery of the air electrode 14 is a layer of a dense, solid electrolyte 16, which is gas tight and dense, but oxygen ion permeable/conductive, typically made of calcia- or yttria-stabilized zirconia. The solid electrolyte 16 is typically about 0.001 mm to 0.1 mm thick, and can be deposited onto the air electrode 14 by conventional thin film deposition techniques such as EVD or plasma spray.
In the prior art design, a selected radial segment 20 of the air electrode 14, preferably extending along the entire active cell length, is masked during fabrication of the solid electrolyte, and is covered by a interconnection 22, which is thin, dense and gas-tight and provides an electrical contacting area to an adjacent cell (not shown) or to a bus bar power contact (not shown). The interconnection 22 is typically made of lanthanum chromite (LaCrO3) doped with calcium, barium, strontium, magnesium or cobalt. The interconnection 22 is roughly similar in thickness to the solid electrolyte 16. An electrically conductive top layer 24 typically nickel plating is also shown.
Surrounding the remainder of the outer periphery of the tubular solid oxide fuel cell 10, on top of the solid electrolyte 16, except at the interconnection area, is a fuel electrode 18 (or anode), which is in contact with the fuel during operation of the cell. The fuel electrode 18 is a thin, electrically conductive, porous structure, typically made of nickel-zirconia or cobalt-zirconia cermet approximately 0.03 mm to 0.1 mm thick. As shown, the solid electrolyte 16 and fuel electrode 18 are discontinuous, with the fuel electrode being spaced-apart from the interconnection 22 to avoid direct electrical contact.
Referring now to FIG. 2, a prior art, very high power density solid oxide fuel cell stack is shown. The cells are triangular solid oxide fuel cells 30. Here the triangular air electrode 34 has the geometric form of a number of integrally connected elements of triangular cross section. The air electrode can be made of lanthanum manganite. The resulting overall cross section has a flat face on one side and a multi-faceted face on the other side. Air O flows within the discrete channels of triangular shape as shown. An interconnection 32 generally of lanthanum chromite covers the flat face. A solid electrolyte covers the multifaceted face and overlaps the edges of the interconnection 32 but leaves most of the interconnection exposed. The fuel electrode 38 covers the reverse side from the flat face and covers most of the electrolyte but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. Nickel/yttria stabilized zirconia is generally used as the fuel electrode which covers the reverse side. Series electrical connection between cells is accomplished by means of an electrically conductive top layer 35 of flat nickel felt or nickel foam combination pads one face of which is sintered to the interconnection while the other face is sintered to the apexes of the triangular multifaceted fuel electrode face of the adjacent cell. An example of a dimension is width 36, about 150 mm and cell plate thickness—about 15 mm. This triangular cell design is active throughout the entire length spanning the interconnection. Fuel is shown as F.
FIG. 3 illustrates a hybrid transitioned solid oxide fuel cell 40 having a triangular, active length 44 having a triangular active cross-section. The inactive cross-section 46 has flattened parallel sides 48, as shown. One out of eight hollow air feed tube 49 is also shown.
Refer now to FIG. 4, which is a standard triangular delta cell, where inlet air is in counter flow mode to outlet air, and which better illustrates the operational aspects of a delta cell. Solid oxide fuel cell 40 has a full triangular active length 44 with a triangular active cross-section. This triangular active cross-section contains a bottom interconnection 45 as well as an air electrode, fuel electrode, interconnection and solid oxide electrolyte therebetween. Channels 62 in the open-face will all contain hollow air feed tubes 49, although one is only shown for sake of simplicity.
Fuel F passes between and outside the triangles as at 65 contacting a fuel electrode on the active outside side of the triangles to provide reacted spent fuel 66 which passes through the separation between adjacent fuel cells. A separate triangular sintered end piece 72 is also shown.
Process air O is fed into the air feed tube 49 and passes to the closed end 74 where it reverses flow, passing upwards in the annular space between the cell cathode and the air feed tube back through the air passage, as shown, while contacting the air electrode within the channels to provide depleted air stream 76 which exits at the open end of the cells, enters a combustion zone (not shown) and reacts with spent fuel.
Referring now to generator prior art FIGS. 5 and 6, prior art design placed very heavy components on top of the fuel cellstack. FIG. 5 (Holmes et al., U.S. Pat. No. 6,656,623—FIG. 2) illustrates, as of its issue date of 2003, standard fuel cell generator structure with a massive insulation board components 86, air inlet plenum 82 with top air O feed 84, air feed tube support board 87 and composite, sliding fuel cell seal and positioning gasket 88, all supported generally by the fuel cells or peripheral components. Also shown are bottom insulation board 86′ as well as bottom fuel distribution boards 90. As shown, SOFC fuel cell bundles are closed end down, 92 showing the closed end, with air feed tubes 94 extending to the bottom of the cell. Fuel F enters at the bottom of the generator as shown.
Also, FIG. 6 (Gillett et al., U.S. Pat. No. 5,741,605—FIG. 3) illustrates, as of its issue date of 1998, prior art fuel cell generator structure 100 with removable module SOFC module stacks, with interior insulation boards 104 and 108. The generator 100 has top oxidant distribution assembly 111 within oxidant plenum 114, certainly massive structures, with equally massive top oxidant inlet 112, all on top of the SOFC area 121, with fuel plenum 116 at the bottom. Interior reformer and recirculation loop are shown as 113.
Now referring to FIG. 7 (Draper et al., U.S. Pat. No. 4,801,369—FIG. 1) illustrates, an unusual design of interconnected solid oxide electrolyzer tubes 125 in an electrolyzer 120 in container 122, where each tube is sealed at the top with a ceramic cap 110 and is open at the bottom, with side power leads 115. Steam feed passes into header 118 to distribution chamber 119 between ceramic boards 130 and 131. Holes in the plates allow steam (H2O(g)) to pass into reaction space 124. Input D.C. power is applied to the anodes and cathodes of the electrolyzer tubes where oxygen is formed inside the electrolyzer tubes 125 passing down chamber 132 and is discharged through header 117. Hydrogen formed is passed out exhaust header 128. This apparatus, completely the opposite of solid oxide fuel cells, adds electricity to dissociate steam (H2O(g)) into H2 and O2 and generates no power. It is a reverse fuel cell with added steam 126 acting as a gaseous separator between oxidant and fuel. This design does not address modularity, does not show how to support cell weight and as a whole lacks provision for internal thermal expansion in any direction.
FIG. 8 (Taylor and Zymboly, U.S. Pat. No. 7,364,812—FIG. 2A) is an inverse fuel cell design with internal fuel reformation assemblies 150 surrounding the fuel cells. Spacer plates 152, 154, 156, 158 and alignment pins 160, 162, 164 form the support structure. The inverted hollow tubular fuel cell 166 contains hollow air feed tube 168. Fuel F enters at opening 170, passes into fuel supply manifold 172 and flows down and reverse flows up reformation assemblies 150 to become reformed fuel F′. Catalysts within reformation assembly 150 can reform feed natural gas. Seal locator strips 174 are used to connect adjacent fuel cell bundles. Air is shown as O with arrows showing its flow patterns. This design does not allow for controlled thermal expansion between bundles during operation, and the internal reformation assemblies take up at least 20 vol. % of the SOFC cross-section and prevents close contact of fuel cells for efficient packaging and electrical distribution. There is no suggestion of reformation in situ on the cell.
All these prior art generator figures are provided to illustrate the current state of the art, and so they, in instances of FIGS. 5-8, can be distinguished from invention of FIGS. 9-15.
As described above, there is a long felt need for a solid oxide fuel cell module and stack design that can compete as an energy source in real world situations, and work in large generators having module bundle rows of up to or exceeding ten bundles. Many have provided proof-of-concept pilot processes where costs are unrealistic in a commercial market. What is needed is a dramatic redesign and rethinking of how the entire to date SOFC generator operates. It is a main object of this invention to provide a departure from previous prototypical, costly, generally non-commercial designs. It is another object to provide a cost effective design for commercial SOFC that has to have revolutionary rethinking to dramatically reduce costs and improve SOFC generator internal volume electrical generation.